1. Field of the Invention
This invention relates to a key operated pin-tumbler lock where a thumb turn key can be retained in the lock and a method of operation therefor. More particularly, this invention relates to a key operated pin-tumbler lock where the thumb turn key can be automatically retained in the lock when inserted and removed by using manually operated release means.
2. Description of the Prior Art
Pin-tumbler locks with thumb turns on the inner side are known. These locks can be a security problem in that the lock can easily be opened from the inside by breaking a window in the door or near the door to provide access to the thumb turn. Pin-tumbler locks that must be opened or locked with a key both inside and outside are also known. These locks can present a problem from a safety point of view in that they can prevent a quick exit in the event of a fire or other emergency. If the lock is locked from the inside but the inside key has been removed, obviously, the door cannot be opened until the key is located. The advantage of this lock is that a burglar who has access to the inside of the lock still is unable to open it without the key.
Pin-tumbler locks having thumb turn keys that are retained on the inside but still removable by taking an additional step, are also known. The Gerlach U.S. Pat. No. 4,120,184 issued on Oct. 17th, 1978 describes a pin-tumbler type of lock having a special abutment portion on the key which prevents removal of the key from the lock cylinder unless a special removal instrument is inserted into the lock cylinder adjacent to the key to release the locking pin from the abutment and permit the removal of the key. The U.S. Oliver U.S. Pat. No. 4,315,420 was issued on Feb. 16th, 1982 and describes a double cylinder lock deadbolt which has a key thumb turn for use with the inside cylinder and a standard key for use with the outside cylinder. When the inside key has been inserted, it triggers a spring-mounted lever which retains the key in the inside cylinder until the outside key is inserted into the outside cylinder and turned to release the inside key from the lever. Previous systems for retaining or allowing a key to be removed from a locking cylinder are either too complex; too expensive; or the manner in which the key is made removable is disadvantageous; or the design does not permit sufficient variations to thwart would be burglars. For example, with the lock described in the Gerlach patent, the key can only be removed through the use of a special removal device. Also, when the inside key is inserted in the locking cylinder and the lock is in a locked position, the lock cannot be opened from the outside using the appropriate key. Also, burglars who are familiar with the Gerlach lock could carry the removal device with them. With the Oliver patent, if one is inside the premises and wishes to remove the inside key from the locking cylinder, one must find the outside key, open the door, insert the outside key in the outside locking cylinder and turn the outside key before the inside key will be removable.